This invention relates to a curved tube type ultra high pressure mercury arc discharge lamp device.
A curved tube type ultra high pressure mercury arc discharge device is often used to form such fine pattern as an IC pattern, for example, on a semiconductor wafer with photolithographic technique for the purpose of minimizing the effect of interference or diffraction of light.
Such curved tube type mercury arc discharge lamp comprises a curved transparent glass tube in which a quantity of mercury is sealed, and tungsten electrodes sealed at both ends of the tube, and a voltage is impressed across the electrodes to emit light through a curved portion at the center of the tube. With this construction, however, it is difficult to seal in the tube a predetermined quantity of mercury. The stability of the current flowing through the tube is determined by the length of the electrodes projecting beyond the surfaces of the mercury, and the life of the tube is also determined by that length. Accordingly, it is necessary to adjust the quantity of the mercury sealed in the tube in accordance with the length of the electrodes projecting into the tube through the surfaces of the mercury. Such adjustment is usually done by adjusting the length of the seals but in a prior art construction such adjustment requires a considerably high degree of skill and is a difficult job. Moreover, in the prior art discharge lamp device, as the stems and electrodes are electrically connected together when the tube is cooled with water, high potential portions are exposed in water to decrease insulating strength.
Further, in a prior art discharge lamp holding device since the stems are electrically connectd to the electrodes and since the stems are soldered to a terminal board secured on a locator so as to mount the discharge lamp on the locator it has been necessary to make the locator of an insulating material. Moreover, as the electrode supports become high temperature, it is necessary to form them with such materials having high temperature resistant property and high electric insulating strength as ceramic or glass filled with a powder of mica.
Such locator is not only expensive but also lacks dimensional accuracy because it is a fired product, thus makes it difficult to set the light source at a correct position. Further such ceramic locator has poor mechanical strength.